High frequency applications require a variety of passive components, which enable reduced circuit size and design complexity. Bipolar and Complementary Metal Oxide Semiconductor (BiCMOS) technologies tend to implement a standard buried subcollector process that implants a shallow (on the order of less than 500 Å from the surface of the semiconductor substrate), very high dose (on the order of about 1015 atoms/cm2 or greater) implant and an epitaxial film grown over the wafer post implantation. A buried subcollector is required and is tuned for the bipolar device. In high-end BiCMOS applications, the buried subcollector tends to be shallower and shallower in order to give the necessary device characteristics for faster bipolar devices in subsequent generations. This causes issues with many of the passive devices that are required for high frequency applications.
A typically prior art method of forming a buried subcollector in a semiconductor substrate is shown in FIGS. 1A-1B. Specifically, FIG. 1A illustrates an initial step in which dopant ions 14 are implanted into a semiconductor substrate 10 using a patterned mask 12 to define an area where said dopant ions 14 are incorporated into the substrate. Reference numeral 16 denotes the subcollector region that is formed in the substrate 10 using this prior art implant step.
FIG. 1B shows the structure after stripping the patterned mask 12 and forming an epitaxial (i.e., epi) semiconductor layer 18 on the substrate 10 including subcollector 16. During growth of the epi layer 18, the dopant ions within the subcollector region 16 are activated and diffuse upwards. Typically, the formation of the epi layer 18 causing all or a majority of the subcollector region 16 to be present in the epi layer 18. The horizontal dotted line in FIG. 1B represents a fictitious interface between the substrate 10 and the epi layer 18. As such, the prior art process does not provide a ‘true’ buried subcollector that remains buried within the substrate.
It has been shown that in 0.25 μm BiCMOS generation, and in subsequent generations, the epitaxial thickness is too thin and the buried implant region is too shallow to build a high performance hyperabrupt varactor diode device, high frequency Schottky barrier diode or a vertical PIN diode. With prior art buried subcollectors used by bipolar devices, it is nearly impossible to create high performance/frequency passive elements.
The high abrupt varactor diode tends to lose merits of performance such as tunability and, in addition, decreases breakdown voltage/increase reverse bias leakage when one implements a later generation BiCMOS NPN buried subcollector process. The Schottky barrier diode cut-off frequency (fc), which is inversely proportional to the devices ‘on’ resistance (Ron) and junction capacitance (Cj), tends to degrade/diminish in subsequent generations due to the scaling of the depth of the buried subcollector. The PIN diode sees the intrinsic region decrease in-depth and clarity with shallower subcollectors.
One of the solutions to the above thinning of the subcollector depth is to implement a dual epi process to create a unique subcollector for the passive devices, which would be deeper than the standard bipolar's subcollector. This adds complexity in the processing due to issues with electrically contacting the deeper unique buried subcollector without additional reachthrough implant levels and, in addition, adds a high cost by adding an entire epi and re-oxidation process for just the high end passive devices.
Another solution that helped to improve the Schottky barrier diode and hyperabrupt varactor breakdown voltage and the tunability of the hyperabrupt diode is the implementation of a unique very deep implanted subcollector implant post the standard buried subcollector epi process. This deep implanted subcollector process has limitations on what implant doses can be used and the sheet resistivity of this process tends to run approximately 100 ohm/square compared to less than 10 ohm/square for the conventional buried subcollector. There is a tradeoff between the energy and dose of this implant—one could increase the dose, but would need to then decrease the energy/depth which then degrades the tunability, or one could implant with higher energy/deeper, but would then need to decrease the dose which tends to degrade the quality factor. This increase in resistance degrades the quality factor of a hyperaburpt varactor at high frequencies due to the lateral resistance increase of this deep implanted subcollector. The Schottky barrier diode observes a significant increase in the Ron with a deep implanted subcollector thus the Fc degrades. Moreover, the tail on this deep implanted subcollector implants eliminate any possibility to build a PIN diode.